Developmental tuning in a spinal nociceptive system: Effects of neonatal spinalization

Recent studies indicate a modular organization of the nociceptive withdrawa l reflex system. Each module has a characteristic receptive field, closely matching the withdrawal movement caused by its effector muscle. In the rat, the strength of the sensory input to each module is tuned during the first postnatal weeks, i.e., erroneous spinal connections are depressed, and ade quate connections are strengthened. To clarify if this tuning is dependent on supraspinal structures, the effect of a complete neonatal spinal cord tr ansection on the postnatal tuning of withdrawal reflexes was studied. The n ociceptive receptive fields of single hindlimb muscles and compound withdra wal reflexes were examined in decerebrate unanesthetized and awake rats, re spectively. Noxious thermal CO2 laser stimulation was used to evoke reflex responses. Neonatal spinal cord transection resulted in a disrupted reflex organization in the adult rat, resembling that previously found in neonatal rats. The receptive fields of single hindlimb muscles exhibited abnormal d istribution of sensitivity not matching the withdrawal action of the effect or muscles. Likewise, the composite nocifensive movements, as documented in the awake rat, often resulted in erroneous movements toward the stimulus. It is concluded that withdrawal reflexes do not become functionally adapted in rats spinalized at birth. These findings suggest a critical role for su praspinal systems in the postnatal tuning of spinal nociceptive systems.